Field of the Invention
The invention relates to a method for stabilizing network operation of an electric power supply network with a first network connection point at a power generator at a first voltage level, and with a second network connection point connected at a second voltage level (higher-level network), and with a number of third network connection points to particular associated loads. It also relates to a device for carrying out the method.
Description of the Background Art
Power supply network can be understood to mean any network area with which voltage measurements, connection points, inverters, and transformers are associated. This can be a local area, such as the premises of a relatively large company, a town, a big city, or a neighborhood of a city, for example. In this way, any desired infrastructure can be defined as a supply network as long as the minimum required components are present. Such a power supply network, which is connected to an associated higher-level network through a transformer, comprises multiple supply levels, namely a low voltage level up to 1 KV, a medium voltage level between 1 KV and 33 KV, a high voltage level from 33 KV to 220 KV, and an ultra-high voltage level above 220 KV, of which two supply levels customarily are linked.
In the practice of public power supply, the power suppliers or network operators set more or less narrow limits for current and voltage quality with respect to the reactive power component (reactive power draw or reactive power delivery) from power suppliers and power consumers in the power supply network, represented by the cos phi value in the phasor diagram of current and voltage, as a function of supply level. In many cases, including in Germany in particular, the permissible value of cos phi for penalty-free reactive power draw or reactive power injection is 0.95. This measure or limit setting serves to stabilize the networks in order to avoid an overvoltage, which can result in destruction of connected loads, and to avoid an undervoltage, which can cause loads to fail. Price serves to regulate adherence to the cos phi values that have been set. Thus, an injection or a draw of reactive power outside of the range for cos phi specified by the power supplier or higher-level network operator is subject to substantial additional charges.
Systems for generating regenerative power are widely known. Every photovoltaic system (PV system) generates direct current, which is converted into alternating current by means of an inverter, and is fed into a power supply network under suitable conditions. Both purely electronic devices and electromechanical converters can be employed as inverters. The term inverter here should be interpreted to mean all devices that can produce an alternating current from a direct current. Thus, although windpower systems directly generate alternating current, this must be adapted to the conditions of the public power supply network through a frequency converter. These frequency converters also include inverters and should be included in the category of means.
The electronic components of an inverter, as is also the case for the combination of a DC machine with a synchronous generator as a mechanical inverter, permit the establishment of a desired cos phi value with respect to power. In most PV systems, this is accomplished by means of a cos phi indicator, by means of which a fixed ratio of delivered power to delivered or drawn reactive power can be established. Thus, the control unit is required to set a cos phi value of 0.97 for all power delivered to the network, for example.
Known from DE 199 61 705 A1, for example, is an inverter for a photovoltaic system by means of which a solar installation can be connected to a power supply network, and which has a control unit for dynamically calculating a compensation current to compensate harmonics and reactive power in the network. The intent is to improve the network voltage quality in the power supply network through operation of the inverter with the control unit. The control unit calculates a desired compensation current value based on a measured network voltage value. In this design, the measuring element for the network voltage element is arranged at the connection point where the inverter delivers the power to the network.
In relatively large and modern systems, the cos phi value is not necessarily fixed, but instead can be adjusted dynamically in operation using a diagram as shown in FIG. 1 depending on the instantaneous requirements. A cos phi value to be established at the output of the inverter is plotted as a function of network voltage there. The output voltage (UNetz) for delivery to the network may only vary within a range between a minimum value (Umin) and a maximum value (Umax). The PV system should not be operated outside this range permitted by the power supplier for delivery to the network with a maximum cos phi of, e.g., 0.95. Within this permissible range lies a narrower range between a minimum control voltage (Uregel min) and a maximum control voltage (Uregel max) in which pure power without a reactive power component can be supplied to the network. The nominal voltage (Usoil) lies in the center of this narrower range.
As a general rule power suppliers, particularly in their contractual relationship to the higher-level network operator (e.g., nuclear power plant, coal-fired power plant, etc.) as electricity vendor, are contractually obligated not to exceed a reactive power draw value of cos phi 0.95, in order to safeguard the voltage stability of the supraregional network. In the example, this means that a cos phi value of 0.94 represents an exceedance of draw, whereas a cos phi value of 0.96 represents a negative exceedance, which is to say an underutilization of the maximum permitted draw. A draw or delivery of reactive power is frequently necessary in the lower-level networks, however, in order to compensate for a voltage rise due to the feed-in of solar and wind power or to compensate for a voltage drop resulting from a dropout in the supply of alternative power generation or from the startup of machinery.
For a more detailed explanation of the problems and the object arising therefrom, an example is discussed with reference to FIG. 2. In a medium voltage supply network at 20 kV, multiple localities A through K of different sizes are supplied with power at an 0.4 KV level through a ring circuit 101 that is connected to two supplying transformers 103 and 103′. The localities A through K are supplied with stable power at the 0.4 KV level. Measures must be taken when one of the transformers, for example the transformer 103′, goes offline because of maintenance work or failure and must be disconnected from the network. The remaining transformer 103 must then supply all localities A through K with power.
In order to be able to provide sufficient voltage even at the distant localities E and D, the control room responsible for the supply network must increase the output voltage at transformer 103. For the nearby localities A and K, this means that they are supplied at the upper edge of the desired voltage range. For the distant localities, this means that they are supplied closer to the lower edge of the voltage range. If a relatively large photovoltaic system is located in the locality K, then its control system will attempt to steer the PV system back into region B—as shown in FIG. 1 described above—since the system is being operated in the area of the right edge, which is to say in the region C, because of the increased network voltage. Since the PV system controller is not aware of the offline transformer 103′, the result is that the system controller will attempt to reduce the network voltage and the control room will attempt to raise the network voltage, for example by appropriate switching actions on the transformers, in particular by a change in the transformation ratio. These opposing interventions in the power supply network have already resulted in less stable networks on a regular basis, even before regenerative power generators were present at today's levels.